In recent years with the use of highly sensitive avalanche photodetectors and higher transmission bit rates, the need for an ultra-fast optical limiter in general optics and integrated optics in particular, for ultimate use in fiber optic systems, has arisen. Such a limiter is a device that protects photosensitive components from optical damage by having a high linear transmittance at low intensities and low transmittance at high intensities. Semiconductors such as silicon, GaAs, InSb, etc. or slower materials such as liquid crystals are obvious candidates. Semiconductors are very attractive since the light induced changes in refractive index can be subpicosecond, thus giving prompt protection to the optical detectors. In a fiber optic system, such an optical limiter can be used to passively remove the pulse to pulse intensity variations on a fiber channel without resorting to complicated and slow electronics.
There have been some demonstrations in the past of self limiters using various nonlinear materials. Thus, for example, in Optics Letters, Vol. 9, No. 7, pp 291-293 (1984), "Nonlinear optical energy regulation by nonlinear refraction and absorption in silicon" by Boggess et al, is described a silicon picosecond nonlinear energy regulator for 1.06 m wavelength. Valera et al also report self switching and limiting in various nonlinear materials in "Demonstration of nonlinear prism coupling", Applied Physics Letters, Vol. 45, No. 10, pp 1013-1015 (1984).
The optical damage susceptibility of self limiters has also been addressed recently in Optics Letters, Vol. 13, No. 4, pp 315-317 (1988), "Self-protecting semiconductor optical limiters", by Hagan et al.
It is recognized that self limiting in nonlinear waveguide geometries is better suited for potential use with fibers. However, these demonstrations all use bulk optics or prism coupling and thus are not directly applicable to fiber optic systems.
U.S. Pat. No. 4,776,658, Oct. 11, 1988, (Normandin), describes a geometry for use in a fiber optic context which has an enhanced performance by utilizing the intrinsic mode coupling characteristics of optical fibers in order to reduce the required drive energy and perform all the common logic functions. Soref and Lorenzo describe, in their article "All-silicon active and passive guided wave components for 1.3 and 1.6 .mu.m" Journal of Quantum Electronics, Vol. 22, No. 6, pp 873-879 (1986), multimode waveguiding in silicon waveguides for use in the near infrared. While the article mentions the possibility of optically controlling the waveguiding properties, it concentrates on the linear electro-optical properties of these waveguides.